Do not activate unused LVM volumes #382
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This prevents udev from accessing not currently used volumes, and also
makes LVM snapshot operations faster (less locking to do).
There is at least one unsolved problem here: inactive volumes do not have
size listed on
lvs
output. There is a band-aid by caching the usage fromthe last time the volume was active, but it isn't enough for many cases (at
the very least - reporting disk usage of a VM that wasn't started
recently).
There may be also other places that assume existing volumes always have
relevant /dev/ node present.
Fixes QubesOS/qubes-issues#6184